A wireless communication system or network covers a geographical area which is divided into radio coverage areas, e.g. referred to as radio cells or sectors. Each radio coverage area is served by an RBS, sometimes referred to as a NodeB or eNodeB, eNB. A reception point is referred to as a set of co-located antennas that provide coverage to one radio coverage area. One RBS may serve one or several radio coverage areas.
Uplink Coordinated Multi Point, CoMP, is a multi-antenna technique that commonly refers to utilising received signals from more than one such reception point, when performing reception of a transmission from a wireless device. This may be compared to the normal case, where only one reception point, i.e. the serving reception point, is used when performing reception for the wireless device.
Signal processing of multi point signal reception becomes quite easy if all reception points are connected to the same RBS since no additional transport of CoMP data is required between RBSs. But sometimes the most favourable cooperation reception point candidates for a particular wireless device are not co-located within the same RBS. When uplink CoMP, hereinafter also referred to as CoMP reception, between reception points that belong to different RBSs is used, the transmission that has been received from cooperating reception points of respective RBSs must be transmitted to the serving RBS.
The usage of multiple antennas, either for transmission or for reception, has a high potential of improving the system performance in terms of capacity, throughput and robustness. For instance, the multiple antennas may be used to transmit/receive different copies of the same information thus increasing diversity and robustness. Alternatively, these antennas can be used to transmit/receive different information through spatial multiplexing. The transmitting and/or receiving antennas can be co-located or distributed, and can even belong to different reception points or RBSs.
Uplink CoMP is a multi-antenna technique where a wireless device's transmitted signal is received and combined using the received signal not only at the antennas of the serving reception point, but also at the antennas of neighbouring, or cooperating, reception points. The selection of which neighbouring (cooperating) reception points to use may be based on e.g. how much power these reception points receive from the wireless device's transmission.
One challenge when using uplink CoMP is that sometimes the most beneficial cooperating reception points are not located within the same RBS. This means that the serving RBS has to request CoMP data from a neighbouring reception point that belongs to a different RBS. An X2 connection between cooperating RBSs might exist but usually over a backhaul with limited capacity or bandwidth. A simple solution is to cater for higher capacity on the X2 interface in order to also be able to send CoMP data between the RBSs. Both X2 and S1 are logical interfaces, i.e. they do not need to be connected directly, e.g. by means of a physical cable. They generally run over Internet Protocol, IP. Even though X2 is an interface for “direct connection” from one RBS to another RBS, in real case, the traffic may be transported along the same backhaul as S1 related traffic. This solution might however be quite expensive or even impossible due to existing infrastructure. Example of other limitations that may exist are too high latency over the backhaul, the serving RBS's capacity to process incoming CoMP data or the cooperating RBSs' capacity to send CoMP data to the serving RBS.
All these limitations add up to a compiled backhaul capacity limitation that is changing rapidly, even from Transmission Time interval, TTI, to TTI. By using this limitation as an input when determining the cooperation candidates, one can decide if the cooperation will succeed or not. However, using this evaluation criterion for selection cooperation candidates, one might end up excluding candidates that could have been beneficial to cooperate with.